The invention concerns a method for fabricating preforms, in particular preforms for power-driven vehicles. The preforms are formed of a natural fiber mat forming a supporting layer with a decorative layer on its upper surface and functional elements on its lower surface. The layers are combined with each other and brought into the desired shape through a second pressing in a form tool. The invention also relates to a method for fabricating the preforms.
In the manufacture of preforms from natural fibers such as, for example, hemp, jute, flax or sisal, fiber mats are used which are interspersed with a thermoplastic synthetic material as a binding agent, or have a binding agent of this kind adhering to their fibers. A decorative material, for example leather, a synthetic leather, a textile fabric, etc., is attached to the upper, essentially visible surface of the preform for reasons of aesthetics and to improve its tactile qualities. The fabrication of the preform is carried out by pressing together and shaping the natural fiber mat with the decorative material in a form tool at melting temperatures of the thermoplastic binding agent. In this process the natural fiber mat is compressed to the extent that it acquires the mechanical properties required of the preform. In addition, however, the decorative material to be combined with the fiber mat is also compressed to such an extent that its tactile and aesthetic effect is impaired. On the other hand, application of too low a mold pressure leads to insufficient compression of the natural fiber mat with the result that its strength characteristics are inadequate.
A further disadvantage of preforms fabricated from natural fibers in the case, for example, of inner coverings for power-driven vehicles is that the natural fibers can absorb moisture and the properties of the preform can be negatively influenced as a result. Apart from this, the natural fibers release unpleasant and in many cases unacceptable odors.
A preform of the above kind made from composite fiber mats for covering the inside of power-driven vehicles is already known from Published, European Patent Application EP 0 730 947 A2, together with a method for its fabrication. This preform has a decorative layer of decorative material on the visible surfaces while at the rear side functional parts, e.g. fixing elements or reinforcing ribs, connected with island-like areas are molded on through a second injection or a second pressing. These covering components are also fabricated in a single pressing process with the disadvantages described above, and the danger also exists of moisture uptake and unpleasant odors.
It is accordingly an object of the invention to provide a preform and a method for its fabrication that overcome the above-mentioned disadvantages of the prior art devices and methods of this general type, which preform has a high practical value through its good mechanical properties, low tendency to emit odors, exclusion of moisture uptake and an unrestricted tactile and aesthetic effect of the decorative material.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method for fabricating preforms, including preforms to be used in power-driven vehicles, which includes:
providing a natural fiber mat intermixed with a binding agent;
compressing the natural fiber mat at a temperature near a melting temperature of the binding agent by use of a mold pressure in accordance with a desired thickness and strength resulting in a supporting layer having an upper surface and a lower surface; and
pressing on a molten binding agent to the lower surface of the supporting layer for forming a sealing layer at the lower surface and pressing on a decorative layer to the upper surface of the supporting layer via a form tool set at a lower pressure than used in the compressing step.
The object is achieved according to the invention with a method in which first of all the natural fiber mat provided with a binding agent is compressed in the region of the binding agent melting temperature by a mold pressure appropriate to the desired strength characteristics and thickness, and the supporting layer of the preform thus obtained is then combined in a form tool at a lower pressure through a second pressing at its lower surface with a molten binding agent forming a sealing layer, and through a second pressing at its upper surface with the decorative layer.
By this method it is possible to fabricate a preform which has good strength characteristics, whose decorative layer has unrestricted tactile and aesthetic qualities, and which as a result of the one-piece molded-on sealing layer prevents the uptake of moisture and release of odors by the natural fibers.
The method according to the invention is also characterized in that the natural fiber mat is either compressed to a flat-surfaced supporting layer to be molded in a later process step, or is compressed immediately to a supporting layer corresponding with the contour of the preform.
According to a further feature of the invention, reinforcing ribs and functional elements are simultaneously molded on during the second pressing for the sealing layer. The binding agent for the sealing layer and the molded on functional elements or ribs corresponds to or is compatible with that used for binding the natural fibers. As a binding agent, thermoplastic synthetic material is used which is applied to the lower half of the form tool as a molten mass through injection molding or by extrusion.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a preform and a method for its fabrication, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.